• Natural: Nitrogen oxides (NO_x) form naturally through lightning strikes, which provide the high energy needed to oxidise atmospheric nitrogen gas (N₂) into nitrogen(II) oxide (NO).
• Man-made: In car engines, under high temperature and pressure, nitrogen oxides form as a byproduct of combustion and are released into the atmosphere.

Catalytic converters typically contain a platinum or palladium catalyst which facilitates the reaction between nitrogen monoxide and carbon monoxide to produce nitrogen and carbon dioxide.

2NO_(g) + 2CO_(g) ➔ N_2(g) + 2CO_2(g)

Two toxic pollutants (NO and CO) react to form harmless N₂ and a less toxic pollutant (CO₂).

Nitrogen oxide pollutants contribute to the formation of acid rain and photochemical smog.

Oxides of nitrogen form acid rain directly by reacting with O₂ and H₂O_(g) in the atmosphere to produce nitric acid (HNO₃). 

Oxides of nitrogen also form acid rain indirectly by catalysing the oxidation of SO₂ to SO₃ which then goes on to reacts with H₂O_(g) in the atmosphere to produce sulfuric acid (H₂SO₄). 

HNO₃ and H₂SO₄ fall to the Earth’s surface as acid rain.

1SO₂ is oxidised by NO₂:

SO_2(g) + NO_2(g) ➔ SO_3(g) + NO_(g)

2NO₂ is regenerated as NO reacts with oxygen in the air:

NO_(g) + $\frac{1}{2}$O_2(g) ➔ NO_2(g)

3NO₂ can further catalyse the oxidation of more SO₂.

Nitrogen oxides react with unburnt hydrocarbons and other volatile organic compounds in the presence of sunlight to produce peroxyacetyl nitrate (PAN), which is a component of photochemical smog.